| Abstract
| - The phenomenon of β-phenyl quenching has been examined by laser-flash photolysis in a series of α-and/or β-substituted ketones 4−8 with similar excited-state characteristics. It is found that α-substitutionmarkedly increases the triplet lifetimes in contrast to β-substitution. The force field calculations for thevarious staggered conformers of ketones 4−6 and 8-syn show that the lowest-energy conformation in allthese ketones has the carbonyl group and the β-phenyl ring gauche to each other. Despite this geometricalrequirement, the longer lifetimes observed are interpreted as being due to the influence of the α-substituenton the rotational freedom of the planar benzoyl moiety as a whole. The experimental results are suggestiveof the attainment of what appears to be a critical geometry for quenching. This scenario may be likenedto Norrish type II reactions, where the α-substituent has long been known to suppress the eliminationpathway and promote Yang cyclization. In addition, we have shown that the diastereomers ofα,β-disubstituted ketones exhibit distinct lifetimes.
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